The present invention relates generally to reinforced bodies known as fiber-reinforced composites and particularly to composites comprising a matrix material strengthened by prestressed glass or glass-ceramic macrofilaments.
The use of strong filamentary reinforcement materials to improve the strength-to-density ratios of organic and inorganic matrix materials is well known. Reinforced products of this type, referred to in the art as "fiber-reinforced composites", typically comprise a plastic, metallic or ceramic matrix material into which has been incorporated a multiplicity of metallic or ceramic, amorphous, polycrystalline or single-crystal reinforcing fibers or filaments. One common example of such a product is a reinforced plastic composite comprising a cured thermosetting plastic resin matrix containing a substantially volume proportion of glass or ceramic fibers as a reinforcing phase.
Where glass and ceramic filaments have been used as a reinforcing phase in such composites the filament diameters have tended to remain small, typically in the range of 1-50 microns. In the "Glass Engineering Handbook", E. B. Shand, pages 431-432, Second edition (McGraw-Hill, New York 1958), J. A. Grant has noted that fibrous glass reinforcement conventionally consists of filaments of 0.00023-0.00075 inches (.about.5-20 microns) diameter, preferably 0.00040-0.00060 inches (.about.10-15 microns) diameter. U.S. Pat. No. 3,788,935, a patent suggesting the use of a combination of large and small filaments to reinforce plastics, utilizes small interstitial fibers of 1-3 microns diameter and large filaments of 10-100 microns diameter. U.S. Pat. No. 4,140,533 suggests the use of alkali-resistant glass fibers 5-50 microns in diameter as a reinforcing medium for cement products.
U.S. Pat. No. 3,732,180 discloses the use of soft glass flakes, particles or sheets of much larger dimension for reinforcement purposes. The glass used is quite soft, and can be worked into fibers after mixing with the plastic by extruding or otherwise shaping the glass-plastic mass. Again, however, the resulting fibers are of small diameter, e.g., in the range of about 0.1-100 microns.
Recent advances in the field of glass fiber-reinforced composites have involved not only improved techniques for the production of strong, defect-free composites but also methods for evaluating such composites in a manner yielding improved strength and fatigue performance data. In a recent Ph.D. thesis entitled "Fatigue Performance Characteristics and Fatigue Life Limitations of Fiber Glass Composites", (August 1978, Case Western Reserve University), at pages 77-82, H. C. Kim has offered some important conclusions relating to the nature of the life-limiting mechanisms of such composites under prolonged cyclic loading conditions. One important finding was that, in properly prepared, well-bonded composites, the strength and surface integrity of the fibers were dominating factors in composite failure due to fatigue under certain modes of stress. Specifically, under cyclic flexural stress or other tension exerted axially with respect to the reinforcing fibers, fiber failure was found to be the first observed event leading to composite failure.